Spores and
Pollen

Pollen and spores are critical parts of the life cycles of vascular and,
because they have very resistant walls, pollen and spores typically are the
most abundant, easily identifiable, and best preserved plant remains in
sediments and sedimentary rocks. Therefore, their presence and abundance in
sediments can be used as a proxy indicator for composition of plant communities
growing at and around a site over both recent and geologic time.

What are Spores and Pollen Grains?

Spores, as referred to here, include the reproductive bodies of lower
vascular plants such as club mosses, horsetails, and ferns. The earliest
occurrences of spores apparently produced by land plants in the fossil record
are in Lower Silurian rocks, slightly preceding the appearance of the the first
vascular plant megafossils (Cooksonia). Pollen grains are the
sperm-carrying reproductive bodies of seed plants, including gymnosperms (such
as conifers and cycads) and angiosperms (the flowering plants). Pollen first
occurs in the Upper Devonian rocks, corresponding to the occurrence of the
earliest fossils seeds (Archeosperma) in North America. Both spores
and pollen have very resistant walls composed of a substance known as
sporopollenin, and the resistance and inert nature of this wall allows
preservation of pollen and spores in sediments under a variety of conditions.
Pollen and spore walls of each plant species bear certain types of sculpture
and have characteristic apertures; these features are among those used to
identify the types of plants represented in a suite of pollen isolated from a
given site. To study pollen grains and spores, a compound microscope must be
used because pollen and spores are very small, typically between 10 micrometers
(µm) and 200 µm (There are 1000 µm in 1 millimeter, and a
millimeter is about equal to the diameter of the head of a pin). Pollen and
spores typically are isolated from sediments and rocks using both chemical and
physical means, and they ultimately are mounted on microscope slides for
examination with the light microscope. For most geological and environmental
applications of pollen and spore analysis, scientists count and identify grains
from each sample using the microscope and generate pollen diagrams of the
relative (percent) and absolute abundance of pollen in samples from a site.

Application to Earth Science Research

Environmental Studies - Studies of human impacts on the
environment commonly have relied on the historic record, which may cover only
the last few decades, to determine the response of plant communities to
environmental changes. By analyzing pollen from well-dated sediment cores
collected at critical sites, it is possible to obtain high-resolution records
of vegetational change with decadal-scale resolution and to document community
changes over the last few centuries and millennia. For example, in the
Everglades of southern Florida, scientists at the USGS have documented the
response of vegetation to changes in water management practices, nutrient
influx from agricultural activities, and disturbances such as road and canal
construction and fire over the last century. Additionally, they have
established the natural levels of variability over the last few millennia.
Comparison of vegetational trends over both time scales is needed to determine
whether human activities really have had significant impacts on the ecosystem.
These types of data are critical for land-use managers in optimizing management
practices and in planning restoration activities.

Biostratigraphy - The evolution of land plants since
Silurian time resulted in great variety of plant types and spore and pollen
morphologies. The relatively broad dispersal and great abundance of pollen and
spores in sediments and rocks throughout this time has facilitated the
development of pollen zonations for use in biostratigraphic work. Using
biostratigraphic techniques, pollen and spore assemblages can be used to
correlate stratigraphic sections several hundred kilometers apart; for example,
spore floras have
been used to correlate coal beds of Pennsylvanian age from Pennsylvania across
the midcontinent into Kansas.

Meet the Experts

Norman O.
Frederiksen, Reston, VA, Cretaceous and Cenozoic biostratigraphy,
paleoecology, and paleoclimatology of the Atlantic and Gulf Coastal Plains and
the North Slope of Alaska.